LONDON (Reuters) - Britain’s Prince Harry and his American fiancée Meghan Markle attended a memorial service on Monday to mark 25 years since the murder of black teenager Stephen Lawrence.

Britain's Prince Harry and his fiancee Meghan Markle arrive at a service at St Martin-in-The Fields to mark 25 years since Stephen Lawrence was killed in a racially motivated attack, in London, Britain, April 23, 2018. Victoria Jones/Pool via Reuters

The 1993 murder of the 18-year-old student exposed racism in the London police and led to a change in the law allowing suspects to be tried twice for the same crime.

The service at St Martin-in-the-Fields Church, Trafalgar Square, was also attended by Prime Minister Theresa May.

Lawrence was stabbed to death at a bus stop in southeast London in an unprovoked attack by a gang of white youths shouting racist abuse.

Five men were arrested in the following months, but charges were dropped due to lack of evidence. Two of the men were later convicted of murder in 2012.

The case became a catalyst for change after exposing deep-rooted failings in London’s Metropolitan Police, dominated by senior white officers in an increasingly multiracial society.

A 1999 report by senior judge William Macpherson said the murder had exposed “institutional racism” in the force and also accused officers of incompetence and a failure of leadership.

“Stephen’s family have honored the promise of his life with everything that they have done in his name - the change they have brought about, and the remarkable things they have achieved,” Prince Harry said.

Prime Minister May said the government would work with the Stephen Lawrence Charitable Trust to establish an annual Stephen Lawrence Day on April 22 of each year.

“We will use this day to encourage and support young people in achieving their dreams, and to reflect on Stephen’s life, death and the positive change he has inspired,” May said.

Britain's Prince Harry and his fiancee Meghan Markle arrive at a service at St Martin-in-The Fields to mark 25 years since Stephen Lawrence was killed in a racially motivated attack, in London, Britain, April 23, 2018. Victoria Jones/Pool via Reuters

Reporting by Guy Faulconbridge with additional reporting by Elizabeth Piper and Andy Bruce; editing by Michael Holden and Mark Heinrich